Our application entitled Breast Cancer Bone Metastasis is focused on the characterization of specific biomarkers (PTHrP(12-48) and IL-8) that we have identified in the circulation of breast cancer patients with bone metastasis. Currently, a barrier to understanding and treating diseases of interest to NIH/NCI, such as breast cancer bone metastasis, is the paucity of sensitive and specific biomarkers. The biomarker profile we have identified and validated contains PTHrP(12-48) and IL-8 and discriminates breast cancer patients with and without bone metastasis with a sensitivity and specificity of (98% and 82% respectively). These molecules have the potential to increase the accuracy of bone metastasis diagnosis in the clinical setting, provide new mechanistic insight into tumor-induced changes in bone metabolism and possibly even improve the assessment of breast cancer bone metastasis patient outcomes. Aim 1 will improve our existing assay and measure PTHrP(12-48) and IL-8 in patient plasma and human breast cancer specimens. Aim 2 will explore the mechanism of action of tumor-derived IL-8 in the regulation of osteoclastogenesis. Importantly, we will also obtain genetic evidence for direct effects of IL-8 on osteoclast formation in vivo and determine if IL-8 expression can rescue the osteopetrotic bone phenotype of RANKL-/- mice. Aim 3 will correlate other clinically relevant parameters such as survival, tumor burden and time-to-next SRE with our existing proteomic dataset, and then independently validate and identify the correlated biomarkers using untested archival patient plasma. The studies proposed will significantly impact the field by providing new information regarding the progression of breast cancer bone metastasis. If successful clinically, PTHrP(12-48) and IL-8 have the potential to be utilized as biochemical markers for the evaluation of breast cancer bone metastasis that may predict breast cancer burden and possibly even patient survival.